As materials of shadow masks to be used in cathode-ray tubes for color television and color display, low-carbon, rimmed, cold-rolled steel sheets have hitherto been employed. However, such low-carbon, rimmed, cold-rolled steel sheets have disadvantages, for example, in that nonmetallic inclusions or carbides in the steel can cause unregulated contours of the mask holes when they are made by photoetching and in that low-temperature annealing performed inevitably to prevent adhesion of flat masks to each other in the final annealing will make the crystal grain size so small that levelling prior to press forming will not thoroughly strain the sheet and stretcher strains tend to develop due to yield point elongation caused by solid-soluted C and N during the press work. Development of these stretcher strains, which deform the hole contours and uneven color development, must be avoided to satisfactorily function the shadow mask.
Another problem with rimmed steels is that annealing at elevated temperatures coarsens the crystal grains, thus again deforming the contours of the mask holes on press working of the shadow masks. To overcome these difficulties, variations of steel compositions and final annealing conditions must be strictly controlled.
In an effort to lessen these disadvantages, low-carbon Al-killed cold-rolled steel sheets with a C content of 0.005% or below are recently coming to use.